1. Field of the Invention
The present invention relates to a heating vessel, more particularly to a, but not limited to, cookware for cooking food, and especially refers to a cookware that heats food via fluid convection such as water or steam.
2. Description of Related Art
A cookware is now commonly seen and used in our daily life, and a user can place food to be heated or cooked inside the cookware, so a cookware is substantially equipped with dual functions of accommodating and heating food.
Generally speaking, a cookware for heating can be categorized to a shallow pan, e.g. a frying pan, a baking tray and a stir-frying pan, and a deep pan, e.g. a boiling vessel; wherein a shallow pan heats via thermal conduction and a boiling vessel heats via thermal convection.
A steamer, as shown in FIG. 4a, commonly used by Chinese is made by a plurality of layers stacked onto each other and heats food via steam convection. A steam vessel, as shown in FIG. 4b, commonly used by westerns is quipped with a multi-layer structure or a soup-pan-liked cookware provided with a net basket.
Materials used for cookware are various, such as copper, iron, aluminum, glass, ceramic, etc. In recent years, a cookware made of stainless steel has occupied most of the market share, mainly because a stainless steel product has a better appearance and durable in use, and is acid-alkali resistant, easy to clean, and has no health related consideration.
But stainless steel itself has his own disadvantages, stainless steel in fact is not a good thermal conductive material, its thermal conductivity is only 16 w/m.k, 4% of that of copper. To overcome such a disadvantage, many patents have disclosed arts about how to improve the thermal conductivity of stainless steel and how to let a heating surface uniformly being heated. As disclosed in the U.S. Pat. No. 6,267,830, wherein a relatively thick copper core having relatively thin layers of pure aluminum bonded between outer layers of stainless steel for improving thermal conduction and processing property. As disclosed in U.S. Pat. No. 5,064,055, wherein a comparatively thick bottom plate of aluminum is provided at a bottom heating surface for improving thermal conduction and distribution.
The above mentioned arts have improved the disadvantage of poor thermal conductivity of stainless steel, but when used in a cookware with a higher perimeter wall, some shortages still have to be overcome.
A cookware heated by a heater is generally heated from its bottom portion, the art disclosed by the U.S. Pat. No. 5,064,055 improves thermal conductivity of a base portion made of stainless steel, thermal energy within the cookware is mainly input from the bottom portion; if the content within the cookware is a fluid, the heating is mainly conducted via convection; the vertical wall of the cookware is made of thin stainless steel thus the thermal conduction alongside the vertical wall is not efficient, the heat conducting ability of the thermal convection of the fluid provided in the cookware is relatively higher. But the stainless steel perimeter wall that conducts heat alongside the perimeter wall is thin, the lateral thermal resistance is even smaller due to the thin size, so the thermal energy within the cookware would radiate to outside; when the fluid within the cookware is heating, the thin vertical wall of the cookware becomes a heat sinking device, the heating efficiency within the cookware is therefore reduced, as shown in FIG. 7.
As shown in FIG. 6, a round cookware that the height thereof and the diameter thereof are the same is served as an example; if the radius of the round cookware is r, then the diameter is 2 r, the height is also 2 r, the heating surface B is πr2, the area of the perimeter wall of the cookware is 2πr times 2 r and is equal to 4πr2. If the heat sinking of the cookware cover is neglected, the heat sinking surface of the perimeter wall is 4 times larger than the heating surface of the bottom portion of the round cookware. Even if the actual situation is more complicated, and may involve the property of the fluid, temperature outside the cookware, the type and the thickness and the surface condition of stainless steel, and the intensity of the thermal source, generally speaking, if the height of the cookware is higher with respect to the diameter of the cookware, the heat sinking condition is more serious, this can be proved by the fact that water in a cookware with higher perimeter wall is harder to reach the boiling point. If the height of the cookware is 1.5 times larger than the diameter of the cookware, the heat sinking surface of the perimeter wall is 6 times larger than the heating surface of the bottom portion of the cookware, in this condition the heat waste is serious and obvious.
If the art disclosed in the U.S. Pat. No. 6,267,830 is adopted, wherein a relatively thick copper core having relatively thin layers of pure aluminum bonded between outer layers of stainless steel; because the good thermal conductivity of the copper layer, the temperature of the copper layer is often higher than that of the fluid inside the cookware, but the outer layer of the cookware is still a negative factor for heat sinking protection; because the temperature outside the cookware is lower than that of inside the cookware, the thermal energy radiated out of the cookware via the perimeter wall is larger than the thermal energy input to the cookware, the thermal energy is therefore wasted, as shown in FIG. 8. So the higher the perimeter wall is, the larger the heat sinking surface is and the more thermal energy is therefore wasted.
The present invention provides a heat adiabatic art that the heat sinking from the perimeter wall of the cookware is prevented and thermal energy is reflected back into the cookware, so the energy efficiency is increased and effects of saving energy and reducing cooking time are achieved. When the energy-saving art provided by the present invention is applied to an electric rice cooker or an electric water heater, the electric rice cooker and the electric water heater can be more compact.